The utility of238Pu produced from237Np irradiation depends on the amount of236Pu present. Calculations of236Pu produced by the (n,2n) reaction on237Np indicate that light-water power reactors produce238Pu containing 10 to 20 times as much236Pu as that produced in the USAEC D2O-or graphite-moderated plutonium production reactors, and the liquid-metal fast breeders produce 300 to 400 times as much236Pu as the production reactors.